This invention relates generally to voltage supplies and more particularly to sub-rail voltage supplies.
Many applications require a voltage that is somewhere between the positive supply voltage (V.sub.cc) and the negative or common supply (V.sub.ss). In many cases this voltage is required to be midway between the positive an common supply voltages. In these cases, the voltage is referred to as a midpoint voltage and the circuit that generates the voltage a midpoint voltage generator. In general, however, these voltage generators can be referred to as "sub-rail" voltage generators because their output voltages are below the positive voltage rail.
An example of a midpoint voltage generator is shown in FIG. 1. The midpoint voltage generator of FIG. I includes a voltage divider network comprised of resistors R1 and R2 and diode-connected transistors Q1 and Q3. The voltage generator also includes an output stage comprised of transistors Q2 and Q4. The voltage divider network establishes two voltages: one at node 10, which is supplied to the base of transistor Q2, and another at node 12, which is supplied to the base of Q4. These voltages bias the respective output transistors to establish a quiescent current flowing through transistors Q2 and Q4. The transistors Q1 and Q2 compensate for the base to emitter voltage drops across transistors Q2 and Q4, respectively. The values of R1 and R2 are typically equal, but do not need to be. If they are equal, however, the voltage generator produces an output voltage V.sub.M that is at the mid-point between the two supply voltages, hence the name mid-point voltage generator.
The output of the midpoint voltage generator is supplied to an output terminal 14, to which a capacitive load C.sub.L and a resistive load R.sub.L is connected. A load current I.sub.L is shown flowing into the capacitive load, which represents the current demanded by the load, although shown as a sourcing current, the output current could be a sinking current as well, depending upon the demands of the load.
There are several problems with this voltage generator. The first is that the output impedance is too high for many applications. The output impedance is approximately equal to the parallel combination of the resistances seen looking into the emitters of Q2 and Q4, i.e., (R1 .vertline..vertline.R2)/.beta.. This high output impedance causes large fluctuations in the output voltage V.sub.M as the load current I.sub.L fluctuates. Therefore, the output voltage V.sub.M is not stable with variable loads or dynamic load currents.
Another problem with the voltage generator of FIG. 1 is that the amount of load current that it can supply is quite limited. The maximum load current (I.sub.LMAX) that the circuit can supply to a grounded load is given by the following equation: EQU I.sub.LMAX =[(V.sub.cc -V.sub.BE)/R1].times..beta.,
where V.sub.BE is equal to the base to emitter voltage drop across transistor Q2 and .beta. is the current gain of Q2. For example, if V.sub.cc is 3 volts, .beta. is equal to 100, and resistor R1 is approximately 60 K.OMEGA. , the load current I.sub.MAX is approximately equal to 330 .mu.A. This amount of drive capability is inadequate for many applications.
An improvement on the basic mid-point voltage generator of FIG. 1 is shown in FIG. 2. This is essentially a special-purpose operation amplifier. The voltage generator of FIG. 2 includes a transonductance (gm) stage 14, which monitors the output voltage V.sub.M and compares it to a voltage at node 16 formed by the voltage divider network of R3 and R4. The gm stage 14 drives a Darlington pair of transistors Q5 and Q6. The transistor Q6 provides additional base drive to transistor Q5 thereby increasing the load current that can be supplied by the voltage generator. It is apparent that the voltage generator FIG. 2 is significantly more complex than the circuit of FIG. 1. In addition, this circuit consumes additional quiescent or stand-by current, which makes the circuit less desirable for low power applications. Moreover, the voltage generator of FIG. 2 may have stability problems with reactive loads due to the high open-loop gain of the opamp in the feedback path of the circuit.
Another approach is shown in FIG. 3. The voltage generator of FIG. 3 includes and additional emitter-follower transistors Q7 and Q8, which provide added base current to the output transistors Q9 and Q10, respectively. This circuit is an improvement over the voltage generator of FIG. 1 in that it provides greater current drive capability. However, it is not suited for low-voltage applications. The additional voltage drop across the added emitter-follower transistor (e.g., Q7) consumes a valuable fraction of the supply voltage. Low power applications typically require a smaller supply voltage since power is equal to the product of voltage and current. A typical supply voltage for these applications is three volts (3 V). The output voltage V.sub.M for a mid-point voltage generator would therefore be approximately 1.5 volts. Thus, the voltage drop across transistors Q7 and Q9 and resistor R5 must be less than 1.5 volts. This may not be possible because the voltage drop across each base to emitter junction (V.sub.BE) Of the two transistors can range from approximately 0.7 V to 1.0 V. Therefore the drop across these two transistors itself can potentially exceed the allowable headroom, this does not even consider the voltage drop across R5 or the tolerance in the supply voltage itself. As a result, the voltage generator of FIG. 3 is not well suited for low power applications.
Accordingly, a need remains for a sub-rail voltage supply with low quiescent current, high load currents, and which is well suited for low voltage applications.